This invention relates to a method for diagnosing rheumatoid arthritis. More particularly, this invention relates to a method for objectively diagnosing rheumatoid arthritis by quantitative determination of the presence or absence of rheumatoid arthritis-associated antibodies in patient sera that react with a recombinant antigen.
Rheumatoid arthritis is a chronic systemic rheumatic disease that affects a significant percentage of the population. Traditionally, it has been diagnosed subjectively through clinical observation and dominant complaints by a patient. P. Lipsky, Rheumatoid Arthritis, in Harrison's Principles of Internal Medicine 1423 (1987). Thus, clinical diagnosis of rheumatoid arthritis is subject to the skill of the diagnostician and the severity of disease symptoms in the patient.
For an objective diagnosis of rheumatoid arthritis, the presence of rheumatoid factor (Rf) in the serum of rheumatoid arthritis patients is routinely determined. Rf has been detected in approximately 70% of patients exhibiting clinical symptoms of rheumatoid arthritis. These patients are thus termed "seropositive." The remaining 30% are classified as having "seronegative" rheumatoid arthritis. Numerous conditions besides rheumatoid arthritis are associated with the presence of rheumatoid factor. Therefore, the presence of Rf does not establish a conclusive diagnosis of rheumatoid arthritis. An objective method of diagnosing rheumatoid arthritis that is more closely correlated with clinical diagnoses than is the presence of Rf in sera is needed. Ideally, such an objective diagnostic test would be quick and easy to perform and would not involve radioisotopes or be invasive to the patient.
Sera from patients with various autoimmune rheumatic diseases contain circulating autoantibodies that are directed against cellular, mainly nuclear, components. E. Tan, 33 Advances in Immunology 167-240 (1982). These antibodies, designated as antinuclear antibodies (ANA), are specific for their respective autoimmune diseases and have been useful as diagnostic aids in clinical medicine. Some of the antigens against which these antibodies are directed have been produced by methods of biotechnology and used in diagnosis of respective autoimmune diseases. R. Michael & J. Keene, Molecular Biology of Nuclear Autoantigen, in 18 Rheumatoid Disease Clinics of North America 283-310 (D. Pisetsky, ed., 1992). Success in developing diagnostic tests against these autoimmune diseases suggests that a similar approach might be fruitful for rheumatoid arthritis.
Sera from rheumatoid arthritis patients have also been found to contain antibodies to cellular components. A precipitin line formed in agar gel diffusion tests when sera from rheumatoid arthritis patients and extracts of certain Epstein-Barr virus-transformed human B lymphocyte cell lines, such as the WIL-2 and Raji cell lines, were placed in adjacent wells. M. Alspaugh & E. Tan, 19 Arthritis and Rheumatism 711-19 (1976). The antibody responsible for the precipitate was found to be of the IgG type and the antigen against which it reacted was a nuclear antigen. Thus, the antigen was termed "rheumatoid arthritis nuclear antigen" or "RANA."
Several problems would need to be overcome before a diagnostic test based on the presence of RANA could be developed. The identity of the antigen is not known. Even if it were known, it occurs in small quantities in cells and would be difficult to purify to homogeneity. Such purity is needed because false positives might result if contaminants were copurified with the RANA, given the extreme sensitivity of serological tests that can be devised to detect small quantities of antigen.